1. Field of the Invention
This invention relates to improvements in the manufacture of air filled cushions. In particular, although not exclusively, this invention relates to air cushions of thin-walled plastic sheet, which can be used as infill or cushioning in the packaging and transportation of fragile articles.
2. Description of Related Art
Conventionally, air filled cushions are manufactured from a roll of plastic tubing. A typical example of a method of manufacturing these cushions is described in International Patent Application WO 01/21391, incorporated by reference herein in its entirety. In this method the plastic tubing is drawn through the machine from a supply roll. The walls of the plastic tubing are separated by drawing the tubing over a separator member. Air is then injected into the space between the wall through a needle, which pierces one of the walls of the tube, the hole left by the needle later being isolated by a heat seal. This process requires careful coordination of the position of the separator member and the air injecting needle, and good control of the air injecting needle so that only one wall of the tubing is pierced.
A different way of manufacturing air filled cushions is to pre-perforate the plastic tubing and blow air into the tube through the perforations. Again, this requires careful coordination of the position of the injection head and the plastic tubing.
Both of these processes are stop-start processes, in that the movement of the plastic tubing through the machine must be halted whilst the cushion is filled. Other references disclosing such processes include U.S. Pat. Nos. 4,049,854, 3,868,285, 3,667,593, and 3,366,523, each of which is incorporated by reference herein in its entirety. The throughput rate of the machines is therefore limited. Furthermore, since both processes inject air into the cushions through relatively small holes, a high pressure air injection system, including an air compressor, can be required.
Various attempts have been made to develop a continuous process for filling air cushions, but have been only partially successful, problems being encountered in a number of areas. The difficulty of injecting air into a moving cushion leads to problems with under and over inflation of the cushions. One solution to this has been to inject air into the cushion using high pressure bursts of air, but this requires a high pressure air injection system and a complex control system. Such systems are described in, for example, U.S. Pat. Nos. 4,017,351, 3,817,803, 6,582,800, 6,659,150, 6,209,286, 5,824,392, 6,410,119 and U.S. Patent Publication No. US 2003-0163976, each of which is incorporated by reference herein in its entirety.
An additional problem relates to the heat sealing mechanism. When the machine must be stopped the heat sealer cools down, and when it is restarted the heat sealer takes a small amount of time to reach operating temperature, so that one or more air cushions can not be formed properly.
Moreover, further difficulties are encountered in conventional machines, including difficulty of machine set-up, particularly in drawing the tubing into and through the machine.